On the overload phenomenon in dynamic Brazilian disk experiments of rocks

The Brazilian disk (BD) test is one of the testing methods suggested by the International Society for Rock Mechanics and Rock Engineering (ISRM) for determining the static tensile strength of rocks. Meanwhile, it is also the only method suggested by ISRM to determine the dynamic tensile strength of rock materials. However, it is worth noting that both static and dynamic tensile strengths of rocks tend to be overestimated using the BD specimen. This can be partially attributed to the overload phenomenon, which is particularly pronounced in dynamic BD tests. In this manuscript, the physical interpretation of the load used in BD test is revised based on the Griffith criterion. To systemically investigate the mechanism and the loading rate dependence of the overload phenomenon for rock materials, the dynamic BD tests under different loading rates were conducted using split Hopkinson pressure bar (SHPB) system. A strain gauge was attached 5 mm off the disk center to detect the failure onset. Then the transmitted wave signal was recorded and processed according to the distance of wave propagation on the transmitted bar and the specimen. The so-called nominal tensile strength and the real tensile strength were obtained through analyzing. The overload phenomenon was then quantitatively evaluated using the pre-defined overload ratio. Additionally, numerical simulations were carried out through the particle flow code (PFC) to observe the failure processes of the disk specimens in microscale. The loading rate dependency was introduced to revise the micro parameters to get a better simulation result. The overload phenomenon and the overload ratio were observed and calculated. The results show that: (1) the overload phenomenon of tensile strength can be obviously observed in the dynamic BD tests, and the overload ratio of the tensile strength logarithmically increases with the loading rate. (2) The overload phenomenon inspected by numerical simulation agrees well with the experimental observation. These results have demonstrated that the overload phenomenon does exist in dynamic BD tests. Its intrinsic mechanism is related to the geometry of specimen and the principle of the testing method based on the experimental and numerical tests. The overload ratio can reach 40% under a high loading rate. It is thus necessary to correct the result from the dynamic BD test to determine the real dynamic tensile strength using the method proposed in this work.